Pressure decay is the simplest leak test available for vapor recovery piping: pressurize the line, close it, and watch whether the pressure holds. The simplicity is real but conditional. Temperature changes, trapped air and instrument behavior all move the gauge too, so the difference between a sound line and a small leak is a matter of method as much as of measurement.
What the test actually measures
The gauge reading is the combined result of every process in the line: real leaks out of the system, gas dissolving or coming out of solution, and the expansion or contraction of the trapped vapor as temperature moves. A field method that ignores temperature will fail good lines and pass bad ones. The method’s job is to hold everything except leakage still enough that the gauge has one story to tell.
Preparing the line before the test
Isolation is the first requirement: the section under test must be closed off from tanks, dispensers and vents, because any open path turns the test into a measurement of everything except the pipe. After isolation comes stabilization, where the line is pressurized and then left to settle so that initial temperature and pressure swings pass before the timed measurement begins.
The stabilization wait is the part crews most often shorten. A test started while the line is still warming or cooling produces a drift that looks exactly like a slow leak, and the resulting search for a fault that does not exist costs more than the wait would have.
Running the timed section
- Isolate the section and confirm the boundary points are closed;
- Pressurize to the agreed test level and note the gauge, time and temperature;
- Allow the stabilization period before starting the timed measurement;
- Record gauge readings at fixed intervals, with temperature alongside;
- Compare the decay against the agreed acceptance criterion;
- If the result is marginal, repeat the test rather than argue from one reading.
Reading the results honestly
A clean, flat gauge line is the easy case. The hard cases are the small drifts, and this is where the recorded conditions earn their keep. A decay that tracks a temperature change points at the environment, not the pipe. A decay that continues uniformly regardless of temperature points at a leak worth finding. Neither conclusion is available if temperature was never written down.
Test elements and what each contributes
| Element | Purpose | What happens without it |
|---|---|---|
| Isolation boundary | Limits the test to the section in question | Result measures the whole system, uselessly |
| Stabilization period | Lets temperature and pressure settle | Drift mimics a leak, false failures |
| Timed readings | Turns the test into evidence | Only a vague impression remains |
| Temperature record | Separates thermal effects from leakage | Marginal results cannot be judged |
| Repeat on marginal results | Confirms or clears a suspect line | Decisions made on a single ambiguous reading |
Finding the leak when the test fails
A genuine failure starts the second half of the method: locating the leak. Section-by-section isolation narrows the range, then joint inspection or listening methods take over. The test record from the first pass helps here, because a fast decay suggests a larger opening, while a barely-failing rate points the search at joints rather than at the pipe body.
Frequently asked questions
Why does the line need to stabilize before timing?
Because pressurizing changes the gas temperature, and the gauge drifts while the line returns to ambient. Timing during that drift measures the temperature change, not the leak tightness of the pipe.
Can pressure decay find very small leaks?
It finds them if the method is patient: longer test times resolve smaller rates. The limit is the instrument and the temperature stability, so improving the method usually beats changing the equipment.
What should be recorded for a defensible test?
At minimum: the test pressure, start and end times, temperatures, the isolation boundary and the gauge readings. A result without conditions cannot be compared to anything later.
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